Key telephone system signalling and control arrangement

ABSTRACT

A key telephone system having a plurality of station sets for connection to first or second telephone lines. The system includes a common control line for transmitting and receiving control signals at each of the station sets. In response to such a control signal, the handsets of at least two of the station sets are connected to the common control line to establish an intercom connection between them. Each of the station sets also includes a transistor load network having a load impedance which is substantially equal to that of the handset. Each network is normally connected across the control line except when the associated handset is connected to that line so that the overall load on it remains substantially constant. This eliminates transients which may cause received errors in the control signals.

CROSS-REFERENCE TO RELATED APPLICATIONS

The following commonly assigned patent application is related to thisapplication:

Title: Key Telephone System Signalling and Control Arrangement

Inventor: David L. Wetzel

Serial No.: 487,295

Filed: 4-21-83

This invention relates to a key telephone system and more particularlyone in which signalling and control are shared between a common controlunit and a plurality of station sets over a common control line.

BACKGROUND OF THE INVENTION

Early key telephone systems utilized separate pairs of lines for boththe voice and signalling paths thereby necessitating upwards of forty ormore individual conductors for the operation of a 6-button key stationset. Utilizing electronic control combined with an intermediateswitching network has enabled this to be reduced to a single pair ofwires for the voice path and one or two additional pairs for power andcontrol signalling. U.S. Pat. No. 3,549,820 of Dieter J. H. Knollman,issued Dec. 22, 1970 and entitled, "Key Telephone Station Concentrator"discloses an early system of this type. U.S. Pat. No. 3,647,980, ofLucian P. Fabiano, Jr., et al, issued Mar. 7, 1972 and entitled,"Switching Network Control without Network Map" discloses an improvedcentral processor controlled key telephone system. More recently,computer controlled switching systems provide many features of the earlykey telephone systems plus more advanced features such as conferencing,call transferring, speed calling, and call forwarding.

In small installations, there is a need for many of the features offeredby these systems while retaining a directly connected central officepowered service which would continue to operate should power to thelocally controlled key telephone network fail.

STATEMENT OF THE INVENTION

Applicant's abovementioned copending application describes a uniquecontrol arrangement for a key telephone system in which control is timeshared between a central control unit and each of the telephone stationsets over a common control line. Such a system provides both voicepaging, intercom, local power, as well as signalling over the commoncontrol line. When the system is in an intercom mode, the network of thetelephone handset is connected directly across and powered from thecontrol line. Connecting the network to the control line results in atransient which may disrupt the signalling. It has been found that byutilizing a transistor amplifier connected to the line for couplingvoice paging signals to the line, which can be rendered inoperative whenthe line is connected in an intercom mode, that this transient can bereduced to a negligible value.

Thus the present invention provides a key telephone system having aplurality of telephone station sets for selective connection to first orsecond telephone lines. The system includes a control unit whichcomprises circuitry for periodically transmitting a framing signal on acommon control line to each of the station sets. The period between eachframing signal being divided into a selected number of time slots. Atleast one of the station sets comprises circuitry for transmitting anintercom signal in one of the time slots for indicating an intercomstate. That set, plus at least one other set, each comprises circuitrywhich is responsive to the intercom signal, for connecting theirtelephone handsets to the common control line so as to establish anintercom state. In addition, a network including a transistor andemitter connected load resistor, is serially connected across thecontrol line in a normally conducting state. The impedance of thenetwork approximating the impedance of the handset. In addition, the setincludes circuitry which is responsive to the intercom signal forrendering the transistor non-conducting so as to disconnect the loadresistor and thereby compensate for the increased load of the handset onthe control line.

BRIEF DESCRIPTION OF THE DRAWINGS

An example embodiment of the invention will now be described withreference to the accompanying drawings in which:

FIG. 1 is a block and schematic diagram of the telephone station setswhich form part of a key telephone system in accordance with the presentinvention;

FIG. 2 is a block and schematic diagram of a control unit which formsthe balance of the key telephone system illustrated in FIG. 1; and

FIG. 3 illustrates typical control signals transmitted by the sets andunit illustrated in FIGS. 1 and 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the key telephone system comprises up to sixtelephone station sets 101, 102, 103, -106 which are connected inparallel to the Tip and Ring T,R of two central office lines L1 and L2.Since the station sets 101-106 are substantially identical to eachother, only station set 101 is shown in detail. As shown in FIG. 2, thecommon control unit 200 which forms part of the system is also connectedto both the central office lines L1 and L2. The detailed structure ofthe key telephone system will be manifest from the following descriptionof its function and operation with particular reference to line L1 whichhas been designated the primary line. Similar operation is available online L2 and hence corresponding components connected to it will beidentified by corresponding reference numbers followed by the letter"A".

Referring to both FIGS. 1 and 2, a multiplexer (MUX) 202 generates acontinuous 3.906 ms pulse framing signal at a 35.156 ms repetition rate.Both framing and control signals (described below) gate the output of a40 KHz generator 203 through analog gate 204 to generate 40 KHz signalbursts which are coupled to each of the station sets 101-106 on acontrol line CNTL. The interval between the framing signals is dividedinto eight 3.906 ms time slots in which the control signals can beselectively inserted. As shown in FIG. 3, the control signals coupled tothe control line CNTL (also comprising 40 KHz signal bursts) mayoriginate with the common control unit 200, any one of the telephonesets 101-106, or selectively, both. In order to distinguish between theframing signal and the balance of the control signals, the duty cycle ofthe latter is half that of the framing signal. The framing signal isused by microprocessors 116 in the telephone sets 101-106 to synchronizeand control the timing for insertion and detection of the controlsignals in the preselected time slots. As mentioned above, line LI hasbeen designated the primary line. Line 2 can be selected as the primaryline for any set through a control signal to the microprocessor 116.This is done by grounding a control input through jumper J1.

When ringing for an incoming call is received on the line L1, it isdetected by a ringing detector 201 in the control unit 200, which inturn sends a signal to the multiplexer 202. Detection of the ringingsignal on line L1 causes the multiplexer 202 to generate control signalsL1 in the time slot 1 and H1 in time slot 7. Normally, the signal L1 isused to indicate an off-hook status on the line L1 while H1 indicates ahold condition. However, the simultaneous presence of signals L1 and H1in the absence of a privacy release signal PR1 in time slot 5, indicatesa ringing condition on line L1.

The ringing generator 110, connected across line L1 in station set 101responds to the ringing signal and generates ringing tones that arecoupled through transformer 111, volume control 112 to speaker 113. Eachof the station sets has its own independent ringing generators connectedacross the lines L1 and L2 in order to meet the requirement that thesets will continue to function should local power and control be lost.When for instance, the station set 101 goes off-hook, its telephone hookswitch 115 connects a ground through contacts 115-1 to themicroprocessor 116. The microprocessor 116 in turn actuates a relay 117which through its contacts 117-1 connects the Tip and Ring T, R of lineL1 to a standard telephone network 118 through hook-switch contacts115-3.

This causes a d-c voltage drop on line L1, which triggers an off-hookline status detector 205 that in turn transmits a signal to themultiplexer 202. The multiplexer 202 stops generating signal H1, leavingonly signal L1 in time slot 1 thus indicating an off-hook condition online 1. Simultaneously, the central office (not shown) detects theoff-hook condition and the ringing signal ceases.

During a ringing state, the status of line L1 and hold H1 signals intime slots 1 and 7 on the control line CNTL are coupled through a 40 KHzbandpass filter 120 to a detector 121 which is gated open by a controlsignal from the microprocessor 116 whenever the set 101 is nottransmitting control signals to the line CNTL. The detected signals arecoupled to the microprocessor 116, which in turn causes the line L1 lamp122 to blink indicating a ringing state. When ringing ceases, the L1lamp is lit in a steady state thus indicating an off-hook state.

Except at the station set which has initially gone off-hook, thepresence of the off-hook line signal L1 provides line privacy undercontrol of the microprocessor 116 which prevents access of any of theother sets to that line. However, this feature can be readily overriddenby actuating the privacy release button PR1 of control buttons 123 whichcauses the microprocessor 116 to generate a privacy release signal PR1in time slot 5. The presence of this signal PR1 allows one of thebalance of the station sets access to the line L1 upon actuation of itsline button L1 of control buttons 123, which causes the microprocessor116 to actuate the control relay 117. This pulse signal PR1 gates theoutput of a 40 KHz signal generator 125 onto the line CNTL throughanalog gate 126 in time slot 5. Concurrently, the signal blocks thedetector 121 so that the output signal from the gate 126 is not receivedby the microprocessor 116.

To place the line in a "hold" state, actuation of the hold button HLD ofthe control buttons 123 causes the microprocessor 116 to generate a holdsignal H1 in time slot 7 for one frame. At the same time, microprocessor116 commences to generate the privacy release signal PR1 in time slot 5.This is necessary to enable any other set to obtain access to the line.The initial hold signal H1 in time slot 7 is coupled through a filter210 and a detector 211 in the control unit 200 to the multiplexer 202which in turn actuates a relay 212 that places a hold resistor 213across line L1. Actuation of the relay 212 also closes ground contacts212-1 which causes the multiplexer 202 to generate the hold signals H1in subsequent time slots. This hold-handshake arrangement enables anyoff-hook telephone set to initiate a "hold" state which results in thecentral control unit 200 connecting the hold resistor 213 across theline L1.

The simultaneous presence of all hold signals, i.e. the line status L1,the privacy release PR1 and the hold H1 signals in time slots 1, 5 and 7causes the line indicating light L1 of lights 122 to blink at a 2 Hzrate. Since the privacy release signal PR1 is present, any one of thestation sets 101-106 has access to the line L1. When any one set goesoff-hook, it is placed in shunt with the hold circuit 213 therebylowering the line voltage even further. This action is detected by linedetector 205 which causes the multiplexer 202 to release the relay 212thereby restoring the line to its normal off-hook state. In response tothe cessation of the hold signal H1, the initiating set will also ceasetransmission of the privacy release signal PR1 in time slot 5 therebylimiting access to the single station set unless privacy release isactuated again by that set.

Local power to each of the station sets is supplied from a highimpedance d-c power supply 220 in the control unit 200 which obtains itsprimary power from the mains. In each of the station sets, this d-csupply voltage is converted by a power converter 130 to provide anoutput voltage V which is used to drive the various elements. The powersupply 220 also provides internal power for the various elements at thecentral unit 200. Should this power fail, a relay 131 at each stationset will be released thereby connecting the set to one of the two linesL1 or L2 depending upon the position of switch 132. This provides basictelephone service to each of the sets during a local power failure.

To operate the key system in a voice paging mode, the voice pagingbutton VP of control buttons 123 is actuated which generates controlsignals in the microprocessor 116 (providing the intercom is not beingused by any of the station sets) that opens analog gate 136 whichconnects a voice paging microphone 135 to transistor amplifier 137, theoutput of which is connected across the control line CNTL. Concurrently,a voice paging signal VP is transmitted in time slot 3 to the balance ofthe station sets. At these sets, the detected VP signal causes themicroprocessor 116 to open an analog gate 138, thus coupling thereceived voice signal through the transformer 111 and volume control 112to the speaker 113. The VP signal also causes voice page lamps VP oflamps 122 to actuate in the balance of the sets. Concurrently, themicroprocessor 116 actuates a voice paging lamp VP of indicating lamps122 at the initiating set. Voice paging assists in alerting a call foranother party, establishing an intercom connection, or providing groupdiscussion.

For operation as an intercom, the intercom control button IC of buttons123 is actuated thereby transmitting an intercom signal in time slot 4(providing the voice paging is not being used by any of the otherstation sets). The intercom signal causes the intercom IC lamps of lamps122 to actuate at the balance of the sets. Concurrently, relay 140 atthe initiating set is actuated under control of the microprocessor 116thereby connecting both its telephone handset microphone 142 andreceiver 143 to the control line CNTL through the network 118 when thehook switch 115 is operated. Actuation of the control button IC at anyof the other station sets 102-106 will result in a similar action sothat the two telephone sets are connected together through the commoncontrol line CNTL thus establishing an intercom connection. Actuatingthe IC button will automatically disconnect that set from either of thelines L1 and L2.

Actuation of relay 140 closes contacts 140-1 to place the network 118across the control line CNTL whenever the hook switch 115 is operated.The network loading on this line CNTL can cause a transient which maydisrupt the 40 KHz signalling. To negate this, relay contacts 140-2simultaneously open the base of transistor 137 which causes it to stopconducting, thereby removing emitter load resistor 149 from across thecontrol line CNTL. The value of this resistor 149 is selected tocompensate for the loading of the network 118 so that this transient isreduced to a negligible value.

To provide on-hook listening or dialing, the input to the receiver 143is connected to a gate controlled amplifier 150 the output of which isconnected through transformer 111, loudness control 112 to the speaker113. Actuation of either line buttons L1 or L2 of buttons 123 will closerelay 141 and close gate amplifier 150 under control of themicroprocessor 116. Should the set go off-hook by actuation of the hookswitch 115, the ground through contacts 115-1 will cause themicroprocessor 116 to release relay 141 and open gate 150. To maintain abalance in the network 144, a resistor 151 is connected across thenetwork 118 through contacts 115-2, in place of the transmitter 142 whenthe hook switch 115 is in an on-hook condition.

What is claimed is:
 1. In a key telephone system having a plurality of telephone station sets for selective connection to first or second telephone lines;a control unit comprising: means for periodically transmitting a framing signal on a common control line to each of the station sets, the period between each framing signal being divided into a selected number of time slots; one of the telephone station sets comprising means for transmitting an intercom signal in one of the time slots for indicating an intercom state; and said one and at least one other telephone station set each comprising means responsive to the intercom signal for connecting their telephone handsets to the common control line; whereby an intercom state is established; a transistor and an emitter load resistor network serially connected across the common control line in a normally conducting state; the impedance of the network approximating the impedance of the handset; and means responsive to the intercom signal for rendering the transistor non-conducting so as to compensate for the increased load of the handset on the control line.
 2. A key telephone system as defined in claim 1 in which said one telephone station set additionally comprises:means for transmitting a voice paging signal in a further one of the time slots for indicating a voice paging state; and means for connecting a microphone circuit to the base of the transistor to couple voice paging signals to the telephone line; and the balance of the station sets each comprise means responsive to the voice paging signal for connecting the common control line to a loudspeaking circuit, whereby voice paging is established. 